Detonation circuit for a vehicle air bag

ABSTRACT

A detonation circuit for an inflation capsule (10) of a vehicle air bag has a capacitor (20) which supplies energy to the capsule during a crash condition. The circuit has a transformer (12), the energy source (20) being electrically connectible to the primary coil (P) thereof by two transistors (30, 48) which assume a conducting state when associated control means (16, 18) determine that a crash condition has occurred. Only when both transistors conduct can the energy source (20) cause a change in the current flow through the primary coil (P), which in turn induces a change in current in the secondary coil (S) to which the capsule (10) is connected. 
     It has been found that by monitoring the voltage and current characteristics in the primary coil (P), the state of the capsule (10) can be monitored. For example, it can be determined whether the capsule has fired, and if so further firing pulses may be suppressed.

FIELD OF THE INVENTION

The present invention relates to detonation circuits for detonating avehicle air bag.

BACKGROUND OF THE INVENTION

In electronic restraining systems in vehicles, e.g. an inflatable airbag to restrain an occupant in the event of a crash, the system normallytakes the form of one or more sensors which detect the deceleration ofthe vehicle and a control unit to monitor signals from each sensor andto supply an electrical signal to detonate electrically an inflationcapsule to inflate the bag if the control unit determines that a crashcondition has occurred. There is a requirement for the system to be ableto trigger the safety function very rapidly in the event of a crash.However, it is also necessary that the restraining system should not beinadvertently actuated, for example in the event that a crash conditionis incorrectly detected or if a short circuit occurs.

It is an object of the present invention to provide a detonation circuitfor detonating an air bag which meets the above requirements andovercomes the aforementioned difficulties.

SUMMARY OF THE INVENTION

The detonation circuit according to the present invention has theadditional advantages that faulty triggering of the detonation capsuleis prevented in virtually all cases in which faults occur in thedetonation circuit. The detonation circuit according to the presentinvention also enables the use of one energy reserve for a plurality ofdetonation circuits in as economical a way as possible. It alsoincreases the security against false triggering, caused by a fault inthe circuit affecting the triggering, such that an additionalmechanical, acceleration-dependent switch need not be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a specific embodiment of the present inventionwill now be described, with reference to the accompanying drawings, inwhich:

FIG. 1 is a circuit diagram of an embodiment of air bag detonatingapparatus in accordance with the present invention; and

FIGS. 2 and 3 are plots of voltage against time for a test switchtransistor and firing transistor respectively of the embodiment of FIG.1.

DETAILED DESCRIPTION

The circuit shown in FIG. 1 has as its object the electrical detonationof an air bag capsule 10 during a crash in which the vehicle to whichthe system is fitted is involved. The inflation capsule is connected tothe secondary coil S of a step-down transformer 12 having a primary coilP and a secondary coil S. The primary coil P of the transformer 12 isconnected in one direction to an energy reserve, indicated generally as14, and to a command release detonation system 16, and in the otherdirection to a timed firing means 18.

The energy reserve comprises a capacitor 20 which is charged up to aworking voltage of 50V from a vehicle battery voltage U_(B) by means ofa conventional DC/DC converter 22. Two decoupling diodes 24, 26 areconnected from the battery voltage U_(B) and from one side of thecapacitor 20 (the other side being connected to earth E) to an outputlead 28 of the energy reserve system F4. A test switch transistor 30 isalso connected to the output lead 28.

The output of the command release detonation system 16 is connected tothe base of the transistor 30. The command release detonation system isconventional, and comprises a first input 32 which conveys signals froma control unit CU₁ when the control unit determines on the basis ofinformation from a sensor (not shown) that a crash condition hasoccurred. A second input 34 leads from a monostable multivibrator (notshown) and both inputs are fed into an AND gate 36. Thus, whencoincident signals are received by the AND gate 36 from the control unitand the monostable multivibrator, a "1" signal is output along lead 38to the base of the transistor, and causes the transistor switch toclose, i.e. to conduct. A high value resistor 40 bridges the emitter andthe base of the transistor.

The collector of the transistor is connected to the primary coil P ofthe transformer 12. A stabilized voltage U_(STAB) is also fed via adiode 42 to the input coil of the transformer 12, and a voltage sensinglead U_(SEN) also leads from the primary coil of the transformer to avoltage monitoring means M_(V). On the other side of the primary coilare connected in series a positive temperature coefficient (PTC)resistor 44, a measuring resistor 46 and a MOSFET 48.

The output of the timed firing means 18 is connected to the gate ofMOSFET 48. The firing means 18 comprises an input 50 which, like input32 of the command release detonation system 16, conveys signals from acontrol unit CU₂ when the control unit determines on the basis ofinformation from a sensor (not shown) that a crash condition hasoccurred. The control unit CU₂ and sensor are different from those usedfor the command release detonation system 16. The other input 52 leadsfrom a monostable multivibrator, and the two inputs are fed into an ANDgate 54 whose output is connected to the gate of the MOSFET 48. Aprimary coil current sensing lead I_(SEN) leads from between the PTC andmeasuring resistors 46, 44 to a current monitoring means M_(I).

It should be noted that further detonation circuits can be connected tothe collector of the transistor switch 30, as indicated by the dottedlead 56. Each further detonation circuit would comprise a transformer,an air bag capsule, PTC resistor, measuring resistor, input coil currentsensing lead, MOSFET and timed firing means, as described above. In thisway, the single energy reserve capacitor can be used to actuate morethan one air bag capsule.

In use, electrical signals from the energy reserve capacitor 20 can onlybe supplied to the primary coil P of the transformer 12 when the testswitch transistor 30 and the MOSFET 48 are rendered conductive by theirrespective circuits, in other words only when both control unitsdetermine independently of each other that a crash condition hasoccurred. However, it is also necessary for the two transistors 30, 48to be conductive simultaneously, otherwise no pulses of current willflow to the primary coil of the transformer 12. When pulses of currentare passed through the primary coil, corresponding current pulses areinduced in the secondary coil of the transformer, and if the inducedcurrent is sufficiently large, detonation of the air bag capsule 10occurs.

FIG. 2 shows a plot of voltage U against time t for the signal from thecommand release detonation system 16, in other words demonstrates thetime for which the test switch transistor 30 is closed. This defines awindow within which the MOSFET 48 must be rendered conductive in orderfor the air bag capsule 10 to be detonated. The pulses of the timedfiring means 18 are lllustrated in FIG. 3, which is a plot of the outputvoltage of the timed firing means 18 with respect to time. Thus, inFIGS. 2 and 3, it will be appreciated that the energy reserve capacitor20 has been successively connected to the primary coil P of thetransformer 12 four times within the defined window period.

When a current pulse is induced in the secondary coil and passes throughthe capsule 10, it has been discovered that by detecting the voltage andcurrent characteristics of the primary coil, information can be obtainedas to the state of the secondary circuit, and in particular the state ofthe detonation capsule 10.

It will be noted that the driving voltage in the primary circuit and thecurrent flowing through the primary coil P are continuously detected. Bydetecting the characteristics of these signals slightly before eachpulse of the MOSFET 48, i.e. before each triggering pulse to be releasedfor the firing transistor 48 and comparing them with a desiredcharacteristic stored in the voltage and current monitoring means M_(V),M_(I), it can be determined whether the inflation capsule 10 has firedas a result of previously supplied energy pulses, i.e. it is possible todetermine whether the detonation circuit has been interrupted or whethera short circuit has occurred within the detonation circuit (for exampleas a result of fusing of the inflation capsule).

If either such case is detected, subsequent timed triggering of theMOSFET 48 can be suppressed immediately, or after a further safetypulse, as desired by the voltage or current monitors M_(V), M_(I). Thisis achieved by the monitoring means M_(V), M_(I) which detect, read andcompare the detected voltage U_(SEN) and the detected current I_(SEN)characteristics. Thus, no further transmission of energy packages iseffected to an air bag capsule 10 which has already been detonated (evenafter short circuiting of the capsule). This is important since it isnecessary to achieve economical usage of the energy in the capacitor 20since this may be used for further firing operations in adjacentdetonation circuits, and the energy stored within the capacitor 20should not be dissipated in a short circuited inflation capsule or in adetonation circuit whose capsule has already been detonated.

The maximum current in the primary input coil of the transformer 12 maybe limited by the firing resistance of the inflation capsule 10, theratio of the number of turns on the primary and secondary coils, thevalue of the measuring resistor 46 and the characteristics of the PTCresistor 44. Preferably, low firing current are chosen on the primaryside by having the number of turns in the primary coil greater than thaton the secondary coil and by appropriate selection of the transistors30, 48.

During crash conditions, the state of the air bag capsule 10 ismonitored using the stabilized voltage U_(STAB). The stabilized voltage,which is conveniently 5V, is utilized so that the characteristics of thesensed voltage U_(SEN) and I_(SEN) can be used to determine the state ofthe detonation circuit. Also, it is possible to test the circuit whenthe test switch transistor 30 is still open.

Also, the size of the voltage from the stabilized voltage U_(STAB)ensures that any energy transferred to the secondary side of thetransformer is small, even if during testing the MOSFET 48 is renderedconductive. Thus, the voltage and current provided by U_(STAB) are notof sufficient magnitude to detonate the air bag capsule 10. Informationfrom the secondary side of the transformer can be gained by detectingthe voltage and current in the primary side, and it is thus possible todetermine the state of the inflation capsule 10, even though no directmeasurement of the capsule characteristics is made.

We claim:
 1. A detonation circuit for supplying electrical energy to aninflation capsule for inflating a restraining device, comprising:anelectrical energy source; connection means for supplying electricalenergy to the capsule from the energy source in response to detection ofa crash condition; a transformer having primary and secondary coils, theenergy source being electrically connectible to the primary coil by theconnection means and the inflation capsule being electrically connectedto the secondary coil; and sensing means for sensing at least onecharacteristic in the primary coil and comparing the sensedcharacteristic with stored characteristics.
 2. A detonation circuit asclaimed in claim 1, further comprising a plurality of switch meansconnected in series with the energy source, each switch means beingclosable independently of the others upon determination of a crashcondition by an associated control means.
 3. A detonation circuit asclaimed in claim 2, wherein each switch means comprises a transistor. 4.A detonation circuit as claimed in claim 3, wherein one of the switchmeans comprises a MOSFET.
 5. A detonation circuit as claimed in claim 1,wherein the at least one characteristic is selected from the groupincluding a voltage characteristic in the primary coil and a currentcharacteristic in the primary coil.
 6. A detonation circuit forsupplying electrical energy to an inflation capsule for inflating arestraining device, comprising:an electrical energy source; connectionmeans for supplying electrical energy to the capsule from the energysource in response to detection of a crash condition; a transformerhaving primary and secondary coils, the energy source being electricallyconnectible to the primary coil by the connection means and theinflation capsule being electrically connected to the secondary coil;and voltage sensing means adapted to sense voltage characteristics inthe primary coil and compare the sensed characteristics with storedcharacteristics.
 7. A detonation circuit as claimed in claim 6, whereinthe connection means is adapted to supply discrete pulses of electricalenergy, the voltage sensing means sensing the characteristics before oneor more pulses.
 8. A detonation circuit as claimed in claim 7, whereinthe voltage sensing means is adapted to suppress the supply of energy tothe primary coil if the detected characteristics are unsatisfactory. 9.A detonation circuit as claimed in claim 6 further comprising currentsensing means adapted to sense current characteristics in the primarycoil and compare the measured characteristics with storedcharacteristics.
 10. A detonation circuit as claimed in claim 9, whereinthe current sensing means is adapted to suppress the supply of energy tothe primary coil if the detected characteristics are unsatisfactory. 11.A detonation circuit as claimed in claim 6, wherein the voltage sensingmeans is adapted to suppress the supply of energy to the primary coil ifthe detected characteristics are unsatisfactory.
 12. A detonationcircuit for supplying electrical energy to an inflation capsule forinflating a restraining device, comprising:an electrical energy source;connection means for supplying electrical energy to the capsule from theenergy source in response to detection of a crash condition; atransformer having primary and secondary coils, the energy source beingelectrically connectible to the primary coil by the connection means andthe inflation capsule being electrically connected to the secondarycoil; and current sensing means adapted to sense current characteristicsin the primary coil and compare the sensed characteristics with storedcharacteristics.
 13. A detonation circuit as claimed in claim 12,wherein the connection means is adapted to supply discrete pulses ofelectrical energy, the current sensing means measuring thecharacteristics before one or more pulses.
 14. A detonation circuit asclaimed in claim 13, wherein the current sensing means is adapted tosuppress the supply of energy to the primary coil if the detectedcharacteristics are unsatisfactory.
 15. A detonation circuit as claimedin claim 12, wherein the current sensing means is adapted to suppressthe supply of energy to the primary coil if the detected characteristicsare unsatisfactory.
 16. A detonation circuit for supplying electricalenergy to an inflation capsule for inflating a restraining device,comprising:an electrical energy source; supply means for supplyingelectrical energy to the capsule from the energy source in response todetection of a crash condition, the supply means being adapted to supplyelectrical energy in discrete pulses; and sensing means for monitoringthe condition of the inflation capsule during a crash condition.
 17. Adetonation circuit as claimed in claim 16, further comprising aplurality of switch means connected in series with the energy source,each switch means being closable independently of the others upondetermination of a crash condition by an associated control means.
 18. Adetonation circuit as claimed in claim 17, wherein each switch meansincludes a transistor.
 19. A detonation circuit as claimed in claim 17,wherein the switch means includes a MOSFET.
 20. A detonation circuit forsupplying electrical energy to an inflation capsule for inflating arestraining device, comprising:an electrical energy source; supply meansfor supplying electrical energy to the capsule from the energy source inresponse to detection of a crash condition, the supply means beingadapted to supply electrical energy in discrete pulses; sensing meansfor monitoring the condition of the inflation capsule during a crashcondition; and a transformer having primary and secondary coils, theenergy source being electrically connectible to the primary coil by thesupply means, and the capsule being electrically connected to thesecondary coil.
 21. A detonation circuit as claimed in claim 20, whereinthe sensing means comprises means for sensing voltage characteristics inthe primary coil of the transformer.
 22. A detonation circuit as claimedin claim 21, wherein the sensing means is adapted to compare thedetected characteristics with stored characteristics and suppress thepulses supplied by the supply means upon detection of unsatisfactorycharacteristics.
 23. A detonation circuit as claimed in claim 21,wherein the sensing means includes means for sensing currentcharacteristics in the primary coil of the transformer.
 24. A detonationcircuit as claimed in claim 23, wherein the sensing means is adapted tocompare the detected characteristics with stored characteristics andsuppress the pulses supplied by the supply means upon detection ofunsatisfactory characteristics.
 25. A detonation circuit as claimed inclaim 20, wherein the sensing means comprises means for sensing currentcharacteristics in the primary coil of the transformer.
 26. A detonationcircuit as claimed in claim 25, wherein the sensing means is adapted tocompare the detected characteristics with stored characteristics andsuppress the pulses supplied by the supply means upon detection ofunsatisfactory characteristics.